Myelin has been consiered to be a simple and relatively metabolically inert membrane. However, recent findings suggest that the myelin sheath may possess both a structural and functional complexity which is as yet incompletely understood. Stability of myelin is of utmost importance which is most likely governed by the interactions between its constituent lipids and proteins. Thus those conditions that may lead to an unstable or metastable myelin should be thorougly investigated. Recent findings on the presence of protein kinase and phosphoprotein phosphatase activities in human CNS myelin, that modify the basic protein (a major component of myelin proteins), may provide a mechanism with a potential to influence the lipid-protein interaction(s). The presence of both these enzymes in myelin could play a role in regulating the degree of phosphorylation of basic protein and perhaps it interaction with lipids. In order to understand the physiological function of phosphorylation/dephosphorylation of basic protein, I have proposed to purify these enzymms, especially the phosphoprotein phosphatase currently under investigation in this laboratory, from isolated myelin (bovine and human CNS) for establishing their characteristics and regulatory properties. For example, cyclic-nucleotide dependency for catalysis may suggest possible hormonal involvement in the regulation of phosphorylation and/or dephosphorylation process occurring within myelin.